The present invention relates to a color image forming apparatus that forms a color image by sequentially forming toner images on an image carrying member, and that is used in the industrial fields of electrostatic recording and electrophotography.
It has been conventional practice, in forming a color image by electrophotography, to repeat electrifying, imagewise exposing, developing, and transferring for every independent color component and form the respective color toner images that are aligned in an overlapping manner on a copying sheet. More specifically, to form a color image, that is, to form respective color toner images of yellow (Y), magenta (M), cyan (C), and black (BK) toners using the light modulated based on the color information data derived from a color document, the above-mentioned process is repeated four times. Such a color image forming system, however, requires that every toner image should be transferred onto a transfer member every time the independent toner image has been developed. Thus, such an image forming system has problems, including a larger copying apparatus, a complicated recording process, and disadvantageously long copying time. Other disadvantages include an inferior color image formed due to misalignment of transferred toner images, since the respective color toner images are independently transferred onto a recording sheet in independent transferring processes.
To overcome such disadvantages, there is a color image forming method wherein a plurality of toner images formed on a common photosensitive member are simultaneously developed in order to perform a transferring process at once. This method, however, still has disadvantages; a toner image previously developed is spoiled when another toner image is developed, or, color balance on a final image is deteriorated because a toner of a former image is blended with a developer for an image later formed.
To overcome such disadvantages, there is proposed a method in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Pat. O.P.I. Publication) No. 144452/1981, wherein developing performed while a photosensitive member is not allowed to come into contact with the turf of a developer layer. In this developing method, the operation of an AC bias applied allows a toner contained in a developer to jump to the photosensitive member, whereby developing is performed on the non-contactive basis.
The principle of image forming method according to the developing method disclosed as above is hereunder described. The flowchart in FIG. 19 shows the change in surface potential on a photosensitive member where positive polarity charging is performed and developing is also performed using a toner of positive polarity. PH represents an exposing area on the photosensitive member; DA, a non-exposing area on the photosensitive member; DUP, an increase in potential caused because the positively charged toner T has been deposited on the exposing area PH in the first developing; CUP, an increase in potential on the exposing area PH due to the second electrifying.
The photosensitive member is subjected to uniform electrifying using a Scorotron electrifier, and endowed with a uniform surface potential E. The surface potential E in the exposing area PH drops to near-zero level by the first image exposing using irradiation of an exposing light source such as a laser, cathode ray tube, and light-emitting diode. The positive bias, of which DC component being virtually equal to the surface potential E on an unexposed area, is applied onto a developing unit, and developing is performed, whereby the positively charged toner in the same developing unit adheres onto the exposing area PH that has a relatively low potential, and, as a result, a first positive image is formed. The potential on the area where the positive image has been formed rises by a DUP due to the positively charged toner adhering thereon. Next, when the second electrifying is performed on the Scorotron electrifier, the surface potential further rises by a CUP, thereby the surface potential E reaches a level approximately same as that of the non-exposing area DA. The surface of photosensitive member endowed with the uniform surface potential E is subjected to the second imagewise exposing to form an electrostatic latent image which is developed in a manner previously mentioned to form the second positive image.
When the above procedure is repeated it forms overlapping forms toner images on the photosensitive member, and a single color toner image is obtained. The color toner image is then transferred onto a recording sheet, thereby fixed by pressure or by heating, to form a color image. At this time, the toner and potential present on the photosensitive member are removed and neutralized correspondingly, in order to prepare next color image forming. In addition, the previously mentioned color image forming method can incorporate the neutralizing process prior to the electrifying process. Furthermore, a common exposure light source or different light sources may be used for respective image exposing.
Incidentally, in a color image forming method, when reproducing color tones by overlapping three primary colors, namely, Y, M and C, the BK component is theoretically deemed unnecessary based on the principle of subtractive color reproduction. However, when reproducing a sharp image such as a character or line drawing, it is necessary to enhance black compared to the three primary colors. For such a purpose, forming black by overlapping the three primary colors is insufficient. This problem is attributable to the fact that the three primary color toners in practical application do not have an ideal absorption wavelength region, and that a minor misalignment is inevitable since strictly aligning toner images of three primary colors is impossible. Additionally, in the additive color reproducing process, there is problem such as insufficient image density attributable to the same reasons. Accordingly, in forming a color image, a developing unit containing a black toner is usually incorporated.
In forming a color image according to electrophotography, two methods are generally available: a normal developing method where developing is performed on an electrostatic image on a photosensitive member using a toner having a polarity reverse to that of the electrostatic image, and a reverse developing method where an electrostatic image is developed using a toner having a polarity common with that of the electrostatic image. The reverse developing method is advantageous in that unlike the normal developing method that necessitates a background of image which is continuously exposed, this method only requires the area where a toner adheres solely exposed. Therefore, strict mechanical precision is not required of the associated optical system and the service life of photosensitive member tends to be longer since the member is less frequently subjected to fatigue, and since the electrical potential on the photosensitive member and the toner adhering thereon are of a common polarity.
Accordingly, a recording apparatus having an exposing light source such as a laser, cathode ray tube, and light-emitting diode often incorporates the reverse developing method. However, forming a color toner image on a photosensitive member in compliance with the reverse developing method incurs the following problem. An area where a toner adhered by a prior developing does not readily allow the imagewise exposing light to be transmitted. Even if such light has been transmitted, the surface potential on the photosensitive member does not decrease because of the potential on the toner image. Accordingly, a toner does not readily adhere onto the member in development later performed. In the additive color reproducing method, similar problems occur since perfect alignment, as well as perfect developing in compliance with the nature of independent latent image, are difficult. Accordingly, a desirable color image is not formed when intending to reproduce various color tones, because of deteriorated color balance or since the image is disturbed around the edge portions.
In summary, in image reproducing the need exists for image processing that eliminates the previously mentioned effects regarding the toner images.
However, in electrophotography, the charge potential on a photosensitive member, charge level on a toner, and amount of a toner adhering onto the member vary depending on ambient conditions or in the course of service life. Accordingly, it is quite difficult to control these factors satisfactorily.
Because of these reasons, a user had to manually adjust color balance among Y, M and C color components by comparing an original document with a resultant duplicate. Accordingly, the user has had to make several duplicates while seeking optimum color balance.